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Doklady Earth Sciences, Vol. 387A, No. 9, 2002, pp. 1063–1066. Translated from Doklady Akademii Nauk, Vol. 387, No. 4, 2002, pp. 528–532.Original Russian Text Copyright 2002 by Kopnichev, Pavlis, Sokolova.English Translation Copyright 2002 by MAIK “Nauka /Interperiodica” (Russia).GEOPHYSICSLithospheric Inhomogeneities and Strong Earthquake Sourcesin the Central Tien Shan RegionYu. F. Kopnichev1, G. Pavlis2, and I. N. Sokolova1
Presented by Academician V.N. Strakhov May 22, 2002
Mapping of the shear wave attenuation field in the
relatively short time period. In total, we processed
lithosphere of the Central Tien Shan region was carried
about 500 earthquake records at the depth of 190–
out on the basis of deep-focus Hindu Kush earthquakes
230 km and an epicentral distance (Δ) of 500–1100 km.
registered at more than 40 digital and analog stations.
The frequency filtration of vertical components was
An intense attenuation stripe related to sources of two
carried out during the analysis of digital records. We used
strongest earthquakes (M ≥ 7.0) in the Tien Shan region
a filter similar to the corresponding FSSS filter with the
since the middle 1970s was distinguished in the west-
following parameters: central frequency = 1.25 Hz,
ern part of the area. It has been established that the
width = 2/3 octave at the level equal to 0.7 of the max-
structure of the lithospheric attenuation field substan-
imum [5]. Analog seismograms were preliminarily
tially changed over 10–20 years. The data obtained can
scanned with the assistance of a wide-frame scanner
be attributed to an intense rearrangement of the fluid
and digitized at the frequency of 40 Hz.
field in the Earth’s crust and upper mantle, relatedamong other things to the preparation of strong earth-
For analyzing the attenuation field characteristics,
we used two parameters: logarithm of the ratio of max-imum amplitudes in S and P waves (S/P), as well as
Recent data on the important role of mantle fluids in
logarithm of the ratio of maximum in the S wave to the
the preparation of strong crustal earthquakes [1–3] indi-
coda level at t = 400 s, where t is counted from the
cate that characteristics of the attenuation field of short-
lapse time (S/c400). Maximal amplitudes were mea-
period shear waves can be successfully used in the
sured during the 10-s interval from the arrival time for
future for long-term and middle-term predictions. Only
longitudinal waves and the ±10-s interval from the
the electrical conductivity of rocks is comparable with
S wave arrival time on the travel-time curve for shear
the attenuation of S waves in terms of sensibility to the
liquid phase [4]. The aim of the present work is to studyspatiotemporal variations of the lithospheric attenua-
In the considered range of epicentral distances for
tion field in the Central Tien Shan and compare them
hypocenters located at the depth of ~200 km, direct P
with the seismicity of the region the last 20−25 years.
and S waves fall onto the M boundary at sufficientlylow angles and intersect the crust at i ~ 46°–49°. (For
The study region (39°–45° N and 73°–81° E) com-
the simplest two-layer model of a medium with the
prises a substantial part of the Central Tien Shan region
S wave velocity in the crust and upper mantle equal to
along with the southern margin of the Kazakh Platform
3.5 and 4.6 km s–1 [6], respectively, the ray angle with
in the north and the northwestern margin of the Tarim
the vertical is equal to i ~ 71°–82°.) Taking into account
S/P values, which can substantially differ even for close
We used the records of deep-focus Hindu Kush
stations, as well as earlier results of the attenuation field
earthquakes obtained by 27 PASSCAL digital stations
mapping for the northern Tien Shan region [7], one may
and 11 KNET digital stations of the Kirgiz telemetric
assume that the S/P parameter mainly characterizes the
network in 1997–2000, as well as in 4 analog stations
attenuation of shear waves at the distance of 30−60 km
(CKM-III) in 1976–1999. The analysis of Hindu Kush
southwest of the relevant stations in the Earth’s crust
earthquake seismograms is convenient, since it allows
(first of all, in its lower part at the depth of 30–55 km)
us to obtain representative experimental material for a
It was shown in [8, 9] that the S coda of Hindu Kush
earthquake records was mainly formed by shear waves
1 Schmidt Joint Institute of Physics of the Earth,
reflected from numerous subhorizontal boundaries in
the upper mantle. In this case, as time elapses, S waves
Bol’shaya Gruzinskaya ul. 10, Moscow, 123810 Russia
on the coda intersect the lithosphere and asthenosphere
at more and more acute angles. Hence, parameter
Fig. 1. Map of the study region. Triangles denote seismic stations. Parameter: S/P (1) weak attenuation, (2) strong attenuation;(3) source zone of the Suusamyr earthquake. Epicenters of large earthquakes in the strong attenuation stripe area: (4) M = 7.0, (5) M =6.0; (6) Talas–Fergana Fault; (7) sources with anomalously high values of the helium isotope ratio.S/c400 characterizes the correlation of S wave attenua-
Figure 2 shows the relationship between average
tion in the crust and upper mantle with distance from
S/P values and average epicentral distance for the stud-
the station (reflected S waves appearing on the coda at
ied stations. It can be seen that parameter S/P for simi-
t ~ 400 s intersect the M boundary at distances of ~10–
lar Δ values can change by more than one order of mag-
nitude (KOPG and TKM2 stations). Confidence inter-val for the average S/P value at the level of 0.9 variesfor different stations between 0.10 and 0.30.
The attenuation is generally weak for the Tarim
massif margin, (except for the westernmost stations
WQIA and KASH (Figs. 1, 2)). This statement is also
valid for the following stations located at the northern
margins of large depressions where following direct P
and S waves intersect the lower crust: USP (Chu
Depression), ANA (Issyk-Kul Depression), and KAI
and NRN (Naryn Depression). Sufficiently high S/P
values were obtained for stations KHA and KUU
(southern margin of the Kazakh Platform), which is
consistent with elevated velocities of P and S waves
here relative to the Tien Shan region [6].
It follows from Fig. 1 that comparatively high S/P
values are observed in the majority of the study region.
Against this background, one can see a strong attenua-tion stripe extending from KASH to TKM2 (minimum
Fig. 2. Dependence of parameter S/P on epicenter distance.
corrected for Δ S/P values correspond to these stations).
Black and white circles denote strong and weak attenuation,
In the southern part, this stripe extends along the Talas–
respectively (Fig.1). The straight line shows a conditional
Fergana Fault, which separates the western and central
boundary separating the regions of S/P values correspond-ing to strong and weak attenuation.
Tien Shan regions, and in the KAZ station area turns to
DOKLADY EARTH SCIENCES Vol. 387A No. 9 2002
LITHOSPHERIC INHOMOGENEITIES AND STRONG EARTHQUAKE SOURCES
the north-northeast. A comparatively strong attenuation
is recorded in the Zaili Fault (station TLG) area and thesouthern margin of the Issyk-Kul Depression (stationsULHL and KAR).
It should be noted that the sources of the two stron-
gest earthquakes in the Tien Shan region during the last
25 years, namely, the Kashgar earthquake of August 23,
1985, (M = 7.0) and the Suusamyr earthquake of August
19, 1992, (M = 7.3), were confined to boundaries of theabove-mentioned attenuation stripe. This stripe also pro-voked the earthquake of January 9, 1997, (M = 6.0) in aregion where, according to instrumental and historicaldata, no events with M > 5.0 had been known before.
Figure 3 shows common envelopes of the S coda for
several stations installed in and nearby the source zoneof the Suusamyr earthquake. The envelopes were con-structed beginning from the maximum in the S wave. Att < 400 s, the envelopes substantially differ in shape and
reveal abrupt bends related to the increase and decreaseof slope. As the numerical simulation indicated [10],these bends resulted from the existence of zones char-acterized by a strong contrast of attenuation in the
Earth’s crust and upper mantle (in the given case, nearthe recording stations).
The envelopes were plotted for two stations over
different time intervals. It can be seen that the codadecay rate in 1992 was higher (relative to 1980) at sta-
tion TORK located at a distance of ~20 km from thesource zone of the Suusamyr earthquake. At the sametime, it substantially decreased in 1991–1992 (relativeto 1976–1977) at station KRSU located approximately35 km southward (farther from the source zone).
Figure 4 demonstrates the dependence of the aver-
age value of parameter S/c400 on distance. It should benoted that the dispersion of this parameter at the studiedstations is substantially (approximately two times)
lower than that for parameter S/P. Relatively lowS/c400 values are observed for the majority of stations.
At the same time, very high values of this parametercorrespond to stations located in the source zone of the
Fig. 3. Common envelopes of the S coda for stations locatedin the source zone of the Suusamyr earthquake and its vicin-
Suusamyr earthquake (AML) and in its vicinity (TORK
ity. (1) Records of stations KRSU and TORK in 1976–1977
and NICH). The envelopes are also relatively steep at
and 1980, respectively; (2) the same in 1991–1992;
stations located up to 60–70 km away from the source
(3) envelope for station KAZ.
Let us note that the very high level of S wave with
lithosphere were previously detected in source zones of
respect to the coda and the impulsive character of this
other strong earthquakes in the Tien Shan region [2, 3].
group at station AML is related to a weak attenuation inthe lower crust located in the south rather than the effect
A detailed mapping of the attenuation field in this
of focusing, since our data show that direct P and S
region was carried out in [11] on the basis of numerous
waves recorded by this station have very high fre-
local earthquakes recorded by a remote highly sensitive
quency spectra relative to the neighboring stations. At
station. Judging from these data, the strong attenuation
the same time, very high values of S/c400 for stations
stripe identified by the authors of this work between the
AML and TORK point to a sharp increase of attenuation
KASH and TKM2 stations was absent during the
in the lower crust and upper mantle as the Suusamyr
1970s. In addition, Figure 3 suggests that the structure
earthquake source is approached. This effect can be
of the attenuation field at KRSU and TORK underwent
explained by the existence of a subvertical, strong
attenuation zone penetrating from the lower crust into
The comparatively fast change of the S wave atten-
the upper mantle. Similar features of the fine structure of
uation field can be related only to a rearrangement of
DOKLADY EARTH SCIENCES Vol. 387A No. 9 2002
anomaly. Data of the Institute of Seismology, National
Academy of Sciences, Kazakhstan suggest that a fairly
vast area of seismic quietness can be distinguished
south of station TKM2. Since 1996, earthquakes with
source depths of ~20 km have been recorded here (it is
known that increase in the share of relatively deep-
focus events serves as an important prognostic indica-tor [2, 15]).
We are grateful to R.T. Beisenbaev for placing at our
disposal analog records from station KUU.
1. Kopnichev, Yu.F., Dokl. Akad. Nauk, 1997, vol. 356,
Fig. 4. Dependence of parameter S/c400 on the epicenter
2. Kopnichev, Yu.F. and Mikhailova, N.N., Dokl. Akad.
distance. Black circles denote the stations located within the
Nauk, 2000, vol. 373, no. 1, pp. 93–97.
source zone of the Suusamyr earthquake and its vicinity.
3. Kopnichev, Yu.F., Sokolova, I.N., and Shepelev, O.M.,
Dokl. Akad. Nauk, 2000, vol. 374, no. 1, pp. 99–102.
the fluid field in the Earth’s crust and upper mantle.
4. Berdichevskii, M.N., Borisova, V.P., Golubtsova, N.S.,
Judging from our previous data [1, 11, 12] and MTS
etal., Fiz. Zemli, 1996, no. 4, pp. 99–107.
data [4], fluid-filled interconnected channels, which inter-
5. Zapol’skii, K.K., Eksperimental’naya seismologiya
sect various tectonic structures existing in the lower
(Experimental Seismology), Moscow: Nauka, 1971,pp. 20–36.
crust of the Tien Shan region. At the same time, the flu-ids can rise along the roots of large fault zones from the
6. Roecker, S., Sabitova, T.M., Vinnik, L.P., et al., J. Geo-phys. Res., 1993, vol. 98, no. 9, pp. 15 779–15 795.
upper mantle into the crust [2, 3, 13]. It should be notedthat very high (submantle) ratios of helium isotopes
7. Kopnichev, Yu.F., Dokl. Akad. Nauk, 2000, vol. 375,
were recorded at the end of the 1980s in groundwater
within the strong attenuation stripe (Fig.1). Previously,
8. Kaazik, P.B., Kopnichev, Yu.F., Nersesov, I.L., and
such ratios were never encountered beyond the areas of
Rakhmatullin, M.Kh., Fiz. Zemli, 1990, no. 4, pp. 38–49.
9. Kaazik, P.B., Kopnichev, Yu.F., and Rakhmatul-
lin, M.Kh., Seismicheskie volnovye polya (Seismic Wave
The sharp change of the attenuation field at stations
Fields), Moscow: Nauka, 1992, pp. 16–26.
KRSU and TORK over 12–15 years points to a migra-
10. Kaazik, P.B. and Kopnichev, Yu.F., Vulkanol. Seismol.,
tion of fluids toward the Suusamyr earthquake source
prior to this event. In addition, the attenuation anomaly
11. Kopnichev, Yu.F. and Nurmagambetov, A.N., Fiz. Zemli,
in the AML station area indicates that the channels
along which fluids ascended from the upper mantle
12. Kvetinskii, S.I., Kopnichev, Yu.F., Mikhailova, N.N., etal.,
were preserved here even within 7–8 years after the
Dokl. Akad. Nauk, 1993, vol. 329, no. 1, pp. 25–28.
Suusamyr earthquake. This statement is consistent with
13. Kopnichev, Yu.F. and Sokolova, I.N., Fiz. Zemli, 2001,
data on the rise of mantle fluids in source zones of
strong earthquakes over several decades after theseevents [3].
14. Polyak, B.G., Kamenskii, I.L., Sultankhodzhaev, A.A.,
et al., Dokl. Akad. Nauk SSSR, 1990, vol. 312, no. 3,
Sources of the two strongest earthquakes in the Tien
Shan region during the last 25 years were confined to
15. Nersesov, I.L., Ponomarev, V.S., and Teitel’baum, Yu.M.,
the high-attenuation stripe. Hence, the next strong
Dokl. Akad. Nauk SSSR, 1979, vol. 247, no. 5, pp. 1100–
earthquake could also take place in the area of this
DOKLADY EARTH SCIENCES Vol. 387A No. 9 2002